1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:
37:
38:

private TextBox[] createTextBoxes()
{
    TextBox[] tb = new TextBox[81];
    for (int i = 0; i < 81; i++)
    {
        tb[i] = new TextBox();
        tb[i].Name = "textBox" + (i + 1).ToString();
        tb[i].Text = (i + 1).ToString();
        tb[i].Top = 30 * (i / 9 + 1);
        tb[i].Left = 60 * (i % 9 + 1);
        tb[i].Width = 50;
        tb[i].Parent = this;
    }
    return tb;
}

private int[] readTextBoxes(TextBox[] tb)
{
    int[] array = new int[tb.Length];
    for (int i = 0; i < array.Length; i++)
        int.TryParse(tb[i].Text, out array[i]);
    return array;
}

private int[] readTextBoxes()
{
    int[] array = new int[1];
    foreach(Control c in this.Controls)
        if (c is TextBox)
        {
            int index;
            int.TryParse(c.Name.Substring("textBox".Length), out index);
            if (array.Length <= index)
                Array.Resize(ref array, index);
            int.TryParse(c.Text, out array[index - 1]);
        }
    return array;
}

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:

private TextBox[,] createSudokuField()
{
    TextBox[,] tb = new TextBox[9, 9];
    for (int x = 0; x < 9; x++)
        for (int y = 0; y < 9; y++)
        {
            tb[x, y] = new TextBox();
            tb[x, y].Name = "textBox" + y.ToString() + "_" + x.ToString();
            tb[x, y].Text = "tb" + y.ToString() + "_" + x.ToString();
            tb[x, y].Top = 30 * (y + 1);
            tb[x, y].Left = 60 * (x + 1);
            tb[x, y].Width = 50;
            tb[x, y].Parent = this;
        }
    return tb;
}

1:
2:
3:
4:
5:

public Form1()
{
     InitializeComponent();
     createSudokuField();
}

1:
2:
3:

tb[x, y].Font.Bold = true;
oder
tb[x, y].Font.Size = 15;